1. Field of the Invention
This invention relates to water heaters, and in particular to an instantaneous gas water heater.
2. Background of the Invention
One of the more important milestones in the evolution of mankind has been the development of delivery systems for on-demand hot water. The advantages and utilities of on-demand hot water are numerous: the ability to wash effectively before eating and at other times during the day has enhanced man's ability to avoid diseases; cooks are able to accelerate cooking chores involving hot water by starting out with pre-heated water; and the task of washing eating and cooking utensils, and other household items, has been rendered more effective and efficient through the use of readily available hot water, thus decreasing disease transmittal. High temperature hot water may be provided at the kitchen sink for immediate use in hot beverage preparation. The very existence of commercial and residential automatic dishwashers depends on hot water heaters. These are only a few examples of hot water heater use--myriad other important uses exist in residential, commercial, and industrial settings.
The most common water heater design comprises a relatively large tank which holds water whose heating may have taken hours. If more hot water is used than is in the tank, the outlet water temperature drops dramatically because office low heating rate of these systems. Since the tank loses heat to the ambient, the water must be re-heated periodically, thus wasting energy. Therefore, currently available tank hot water systems may run out of hot water, and waste energy.
One solution to the problems associated with tank hot water heating systems is the instantaneous hot water heater. Instantaneous water heating systems heat water at a much higher rate than tank systems, thereby avoiding the need to store hot water in a tank. Instantaneous water heating systems transfer heat to water only when a demand for hot water is sensed, and then heat the water at a rate sufficient to provide hot water at the same rate as the hot water is being consumed.
Existing Designs
A number of patents have been granted for instantaneous water heaters. U.S. Pat. Nos. 4,604,515, 4,808,793 and 5,020,127 were granted to Davidson, Hurko and Eddas et al. respectively. While these water heaters provided on-demand hot water without the use of a hot water storage tank, these designs incorporated electric heating elements to provide the heat with which to heat the water. Therefore these designs suffered from the problem that natural or low pressure ("LP") gas could not be used to heat the water. In many areas gas is the preferred energy source for water heating due to its low cost and clean combustion characteristics. In addition, electricity is not available in some locations, rendering the use of these electric designs impossible.
U.S. Pat. No. 4,550,689 was granted Wolter for a Gas Instantaneous Water Heater. While this design used gas as the energy source with which to heat water, it suffered from a number of disadvantages. First, the design incorporated three atmospheric burners. The atmospheric burner is the traditional design of choice for gas heaters. In this type of burner, the gas is injected via an orifice into the opening of a venturi, inspirating the air required for combustion to the burner. The air is proportional to the input rate of the burner. Atmospheric burners are used in applications where the back pressure from the flue gasses is minimal and the venturi is capable of providing enough air flow for combustion. However, atmospheric burners suffer from efficiency decreases where the combustion chamber is enclosed and the heated surface is very close to the burning surface (which is the situation in most instantaneous gas water heaters). Under these conditions, a relatively large back pressure is developed by the flue gases, and the venturi used in atmospheric burners is not capable of overcoming the back pressure. In these situations, powered burners are quickly becoming the system of choice.
In a powered burner, the air is supplied to the burner by a mechanical means such as a fan or blower. The current low price and availability of blowers render the powered burner a competitively priced design. The efficiencies of powered burners are higher than these of the atmospheric burner due to increased flue gas velocities. And the design of heat exchangers incorporating powered burners can be more flexible by eliminating concerns of restricting the combustion air.
Second, the multiplicity of burners (three) and serpentine coilings render the Wolter design complex, with attendant cost impact. Finally, the water to be heated does not completely surround the burners, thereby causing energy loss between the coils, negatively affecting heating efficiency.
U.S. Pat. No. 4,501,261 was granted Tsutsui et al. for art Instantaneous Gas Water Heater. While this design used gas for energy, the design was rendered complex through the use of a fan damper which variably adjusted the size of an air supply opening to the combustion chamber, through the incorporation into the design of variable pitch spacing fins on the outer surface of the heat exchange pipe, and through the use of a water flow sensor comprising a vane wheel, magnet and sensor element. These complexities contributed to the cost of the unit. In addition, this design used a two dimensional burner, which only burned gas in a horizontal plane. Therefore the heat transfer efficiency was not as great as if a three dimensional burner were used. The heat transfer rate was further compromised because this design employed only a single trip by the hot air past the heat exchange pipe. Finally, no heat absorbing layer was disclosed on the heat exchanger pipe or fins, thereby reducing the heat transfer efficiency.